PDPAs containing longer (octadecyl, C18) and shorter (methyl, C1) alkyl side chains in the same backbone are synthesized. The copolymerization of C1 and C18 monomers at a range of feed ratios afford high-molecular-weight (Mw > 106) copolymers almost quantitatively. As the content of longer alkyl chain units in the copolymers increases, the emission band in the bulk film shifts to shorter wavelengths and the emission intensity increases. The lamellar layer distance increases with increasing C18 content. The excited species with shorter lifetimes decrease gradually with increasing C18 content, whereas the longer-lasting emission increases. Fine tuning of the FL emission of PDPAs is achieved by simple copolymerization of monomers with different alkyl side-chain lengths.